OSB Subfloor Over Decking?

Q.We’re building a
four-season addition over an existing raised deck
framed with PT lumber. Is it okay to install OSB
subfloor sheathing directly over the wood decking, or
should the decking be removed first?

A.Bryan Readling, a senior
engineer with APA/Engineered Wood Association,
responds: If the deck footings, framing, and
house connections are adequate for the anticipated
loads, and the existing decking is sound and
reasonably flat, you don’t necessarily
have to remove it before laying down a new OSB
subfloor and framing the addition. However, there
are some advantages to doing so anyway.

For one, removing the decking would allow you to
thoroughly inspect the framing and easily make any
needed repairs or upgrades. You’ll also
avoid squeaky floors, bumps in the floor sheathing,
and other problems caused by loose, warped, or
misaligned deck boards, or by wet joists drying out
and causing decking fasteners to lose their grip.
If you take off the decking, clean the tops of the
joists with a scraper, and glue and nail the
sheathing directly to the joists, you will end up
with a solid floor system and be more likely to
avoid callbacks.

If you still choose to install the sheathing
over the existing decking, be sure to follow
APA’s recommended nailing schedule for
underlayment (the Engineered Wood Construction
Guide, Form E30, is available at apawood.org).
Fasteners — ring-shank nails are best
— for 7/16-inch OSB should be 6 inches
on-center around the panel perimeter and 8 inches
on-center each way in a grid pattern on the
interior of the panel. (If you’re using 2
3/32-inch T&G sheathing, the perimeter
nailing schedule is the same, but the grid pattern
can be increased to 12 inches on-center.)

To minimize squeaking and nail pops caused by
shrinking lumber, use construction adhesive and
nail into the decking rather than into the joists.
Also, select fasteners that are long enough to
protrude slightly below the bottom of the deck.
Keep in mind, too, that the deck framing will now
have to comply with local building-code guidelines
for ground clearance, floor insulation, and
ventilation.